PET is a method of administering a compound that is labeled with positron-emitting radionuclides and imaging the distribution of the compound in the body as a tomographic image by processing data obtained by PET detectors including combinations of light receiving elements and scintillators, or CdTe, CZT, or other semiconductor detectors. While X-ray CT (Computed Tomography) and MRI based tomographic images provide anatomical information, PET images are referred to as functional images which express biological functional information. For example, PET images are capable of highly sensitive tumor detection though not precise in position. To add accurate positional information to a PET image, an X-ray CT image or MRI image which can provide exact positional information needs to be superposed on the PET image. For higher superposition accuracy and efficient scanning, PET/CT scanners that combine a PET scanner with an X-ray CT scanner have been prevalent. However, since X-ray CT typically has an exposure level several times higher than that of PET, CT exposure is not negligible.
Instead of the CT scanner, MRI scanners capable of acquiring anatomical images without radiation exposure are receiving attention. PET/MRI scanners that can simultaneously acquire a PET image and an MRI image have been under research and development (see Patent Literature 1 and Non-Patent Literature 1).
PET detectors are composed of scintillators which emit light in response to incident of annihilation radiation and light receiving elements which detect the emitted light. In past approaches, the scintillation light has been brought out to a location less affected by a magnetic field through optical fibers or the like before received by the light receiving elements. Such approaches deteriorate the PET performance because of attenuation of the scintillation light. A semiconductor light receiving element method has recently been developed in which, as shown in FIG. 1, all the detector units (hereinafter, referred to as PET detectors) 10 of the PET scanner are arranged within the static magnetic field of the MRI scanner 8, using APDs (Avalanche Photodiodes) or Geiger-mode APDs (also referred to as SiPMs) less susceptible to a magnetic field as the light receiving elements. Small animal PET scanners and head PET scanners have actually been developed (see Non-Patent Literatures 2 to 4 and Patent Literatures 2 to 4).